Two people who come as a pair are one invite, not two. Sam & Lee get one link, one message, and one answer that counts for both.
Add the second name with Add to party when you add the person, and the invite addresses both of them. An answer counts everyone named on it.
Plus-ones are separate: None, One each, Two each or No limit. Every event starts at None. Set them under More when you make the event, or later under Actions, then Options.
Tick Ask how many kids in Options and guests who are coming say how many children they are bringing, which is the number you need for food and chairs.
A household counts as one invite toward the free 30.
Your People list keeps the household, so the next event you make already knows Sam comes with Lee. Change it with Edit, then Party, on their page in People.